Studies elucidate cause and pathogenesis of chronic degenerative CNS disorders with emphasis on MS, ALS, parkinsonism-dementia, Parkinson's, Pick's, and Alzheimer's diseases, Huntington's chorea, supranuclear palsy, other presenile dementias, chronic encephalitis with focal epilepsy, muscular dystrophies, chronic schizophrenia, SSPE, PML, dialysis encephalopathy, and intracranial neoplasms. Even familial, apparently hereditary diseases may be slow virus infections. Subacute spongiform virus encephalopathies (Kuru and Creutzfeldt-Jakob (CJD) diseases of man; scrapie and mink encephalopathy are caused by unconventional viruses with unique properties posing important theoretical problems to microbiology and molecular biology; a major goal is elucidation of their structure and mechanisms of replication. Transmissible virus dementias are increasingly recognized worldwide causes of death: high incidence foci, transmission by corneal transplant, and occupational hazards from exposure to human brain in surgery or pathology are found. Scrapie transmitted to primates causes disease indistinguishable from experimental CJD. Zonal UC, electrophoretic, chromatographic purification, UV and ionizing radiation inactivation, EM freeze-fracture membrane studies on scrapie are under way. Finding many latent oncogenic viruses, including reverse transcriptase producing oncornaviruses in healthy primate brains and slowly developing noninflammatory pathogenic effects from persistent, masked and defective infection suggest relationships to the cancer problem.